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It’s a bird ... it’s a plane ... it’s a super snowstorm

Here in Colorado, we have all watched a rainstorm across the street leave us completely dry. It can rain heavily in one part of the valley, but not in another. Like my dad always said, “If you don’t like the weather around here, wait 10 minutes.”

In December of 2007, there was a particularly unusual snowstorm on the Front Range not far from Denver International Airport. It lasted 45 minutes and was about 20 miles long, but only 2 1/2 miles wide. It covered the ground with 2 inches of snow. What could cause such a long, skinny snowstorm?

Snow is made of water. Water molecules are arranged in the shape of a “V.” The oxygen is at the point of the “V,” and the two hydrogen atoms are at the ends of the arms. The three atoms do not share electrons equally. The oxygen has all of the electrons most of the time, and that leaves the oxygen with a net negative charge. The hydrogen atoms seldom have the electrons, so they are positive most of the time. That means the water molecule has a slight polarity: a positive end and a negative end. The situation is sometimes reversed momentarily, so this is called a “dipole moment” (two poles changing from moment to moment).

This quick change in polarity means that water tends to cling to itself with the positive ends being attracted to the negative ends. As water molecules cool, they settle into positions aligned positive to negative and get closer and closer together. At about 4 degrees C, the molecules are packed as close as they can be, and the water is as dense as it can usually get. During freezing, the water forms crystal bonds, which actually push the molecules slightly farther apart. At this point the water (ice) becomes a little less dense and this is why ice floats.

Water can sometimes cool below the freezing point without forming an ice crystal. A surface to adhere to is needed in order to form the crystal. Hence, water can sometimes remain liquid with temps as low as -40 C.

However, in that super-cooled state, even slight disturbances can cause the molecules to suddenly crystallize. If there is a dust particle, or another ice crystal, or sometimes even air movement, pushing the molecules together can cause the formation of ice crystals.

Ice crystals formed at a high altitude become heavy, causing the growing crystal to fall through the clouds below leaving behind a hole. These are called “hole-punch clouds.” The falling ice crystal forms the nucleus for water vapor in the clouds below and the crystal grows even larger. These crystals are renamed snow.

Andrew Heymsfield, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, discovered the freak, 2 1/2-mile-wide snowstorm in such a hole-punch cloud. It was created by the movement of a turboprop jet through a super-cooled cloud, on its approach to DIA. While reviewing images taken from a plane-mounted camera, they noticed a long, thin hole in an otherwise solid layer of clouds.

Beneath this hole, a wall of falling snow extended from the hole all the way to the ground, and the hole lined up perfectly with the flight path for planes approaching DIA.

As the moisture-rich air passed the front facing edge of the plane’s propellers, the air accelerated, its pressure decreased, and the temperature dropped about 10 C. Ice crystals were formed. These sucked up surrounding cloud water to gain enough weight to fall through more super-cooled air below, triggering a snowstorm below the plane’s trajectory.

Cloud seeding has been used for a long time, but apparently airplane movement alone can substantially increase precipitation as the plane flies through super-saturated clouds. Maybe that’s why the weather changes so frequently in Colorado ... too many airplanes!